In coating for steel plates and plastics of automobiles, home electric appliances and the like, for the purpose of shortening of coating process and energy saving, there have been a widely adopted coating method called a wet-on-wet system, or a two-coat one-bake system, occasionally, a three-coat one-bake system. In these coating systems, for the purpose of reducing volatile organic compound (VOC) reduction at the same time, for a paint used in a base coat layer of the two-coat one-bake system, and further for a paint used in a surfacer layer of the three-coat one-bake system, low VOC paints, for example, aqueous paints are often used.
However, since paints used in the aqueous base coat layer and aqueous surfacer layer have not reached the level of two-component reactive coated film in terms of coated film performance, there occur problems such as poor adhesiveness, solvent resistance, and water resistance, so methods for improving these drawbacks have been required.
In Patent documents 1 and 2, there is described a method in which a paint composed of a resin containing hydroxyl and carboxyl groups and a catalyst is coated as a base coat, next, a clear coat consisting mainly of a hydroxyl group-containing resin and a polyisocyanate compound is coated so that the isocyanate compound in the clear coat paint penetrates partly into the base coat layer to harden.
Patent document 3 describes that aiming at a similar effect of transferring an isocyanate compound in a clear coat partly into a base coat, as a hardener of the clear coat, a polyisocyanate containing 1 weight % or more, 70 weight % or less of dimer (also called uretdione) of 1,6-hexamethylene diisocyanate (hereinafter, also referred to as HDI) is used in a range of 1.2 to 3.0 in NCO/OH ratio.
In Patent documents 4 and 5, there is described a method for obtaining a water-dispersible polyisocyanate by reacting alkoxypolyalkylene glycol or a compound having an active hydrogen and a sulfo group with polyisocyanate.